Feeding habitat selection in the threatened Olrog's Gull Larus atlanticus breeding in northern Patagonia, Argentina

SUÁREZ, N.; KASINSKY, T.; RETANA, M.V.; MARINAO, C.; YORIO, P.

Ciudad del Cabo

Congreso; II World Seabird Conference; 2015

The Olrog's Gull Larus atlanticusis a threatened species which feeds during the breeding season almostexclusively on three intertidal crab species. We studied the feeding habitatselection during the incubation and early chick stage of gulls breeding at twocolonies (Islote Jabalí and Banco Nordeste) located in the Bahía San Blasmarine protected area, Argentina. Feeding areas were identified by GPS trackingof 67 birds (414 foraging trips). Potential feeding areas were classified usingdominant substrate and structural characteristics and prey characteristics wereassessed by randomly sampling 3,638 1 m2 quadrates. Habitatclassification was conducted using Landsat TM satellite imagery, complementedby a supervised classification using 80 randomly located reference plots in thefield. The patterns of feeding habitat selection throughout the breeding seasondiffered between the two colonies. Birds from Islote Jabalí selected habitatscharacterized by muddy substrates with vegetation and high densities of Neohelice granulata during incubation,but switched to structured habitats (mainly oysters beds) dominated by thesmaller Cyrtograpsus altimanus. Incontrast, birds from Banco Nordeste selected habitats with muddy substratesthroughout the breeding cycle. Crab size differed between foraging areas usedby birds from the two colonies, being the sizes of N. granulata significantly smaller in the Banco Nordeste foraginggrounds. Previous studies showed that prey delivered to chicks weresignificantly smaller than those use by adults for self-feeding. The selectionof coastal habitat by Olrog's Gulls appears to be determined by the seasonalchange in trophic requirements of adults and chicks, given the spatialdistribution of prey sizes.